Abstract
Determination of the true height of specific ionospheric plasma frequencies has been used for finding actual motions within the ionospheric plasma. The reported vertical soundings reveal a periodic variation in time at all bottomside altitudes. Spatial variations of this periodic structure have been observed at points separated by about 50 km. Daytime and nighttime periodic structures are different. Periodic variations of angle-of-arrival data of an oblique propagation experiment on a 1330-km circuit also have been analyzed. These periodicities are related to the same spatial periodic electron density structure in the bottomside ionosphere. The oblique reflections had dominant azimuth periods in the order of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\frac{1}{4}</tex> , <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\frac{1}{2}</tex> , and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1 h</tex> .
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